Vanadium oxysulfide based cathode materials for rechargeable battery

ABSTRACT

A cathode active composite containing a nanoparticle composite of an amorphous matrix containing vanadium, oxygen and sulfur and crystalline regions of vanadium and oxygen embedded in the matrix is provided. Electrochemical cells and a reversible battery having a cathode containing the cathode active composite is also provided. In specific embodiments the battery is a magnesium battery.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of prior U.S. applicationSer. No. 14/693,235, filed Apr. 22, 2015, the content of which isincorporated herein by reference in its entirety. U.S. application Ser.No. 14/693,235 is a continuation in part of U.S. application Ser. No.14/260,544, filed Apr. 24, 2014, now U.S. Pat. No. 9,711,793, issuedJul. 18, 2017, the content of which is incorporated herein by referencein its entirety.

BACKGROUND OF THE INVENTION

The present invention is directed to a positive electrode activematerial for a magnesium secondary battery and a magnesium battery witha cathode based on the active material.

Lithium ion batteries have been in commercial use since 1991 and havebeen conventionally used as power sources for portable electronicdevices. The technology associated with the construction and compositionof the lithium ion battery (LIB) has been the subject of investigationand improvement and has matured to an extent where a state of art LIBbattery is reported to have up to 700 Wh/L of energy density. However,even the most advanced LIB technology is not considered to be viable asa power source capable to meet the demands for a commercial electricvehicle (EV) in the future. For example, for a 300 mile range EV to havea power train equivalent to current conventional internal combustionengine vehicles, an EV battery pack having an energy density ofapproximately 2000 Wh/L is required. As this energy density is close tothe theoretical limit of a lithium ion active material, technologieswhich can offer battery systems of higher energy density are underinvestigation.

Magnesium as a multivalent ion is an attractive alternate electrodematerial to lithium, which can potentially provide very high volumetricenergy density. It has a highly negative standard potential of −2.375Vvs. RHE, a low equivalent weight of 12.15 g/eq and a high melting pointof 649° C. Compared to lithium, it is easy to handle, machine anddispose. Because of its greater relative abundance, it is lower in costas a raw material than lithium and magnesium compounds are generally oflower toxicity than lithium compounds. All of these properties coupledwith magnesium's reduced sensitivity to air and moisture compared tolithium, combine to make magnesium an attractive alternative to lithiumas an anode material.

Production of a battery having an anode based on magnesium requires acathode which can reversibly adsorb and desorb magnesium ions and anelectrolyte system which will efficiently transport magnesium ions.Significant effort in each of these areas is ongoing in many researchorganizations throughout the world and active materials underinvestigation include sulfur in various forms, including elementalsulfur, materials known as Chevrel compounds of formula Mg_(x)Mo₆T_(n),(wherein x is a number from 0 to 4, T is sulfur, selenium or tellurium,and n is 8) and various metal oxides such as MnO₂ (alpha manganesedioxide stabilized by potassium), V₂O₅ and ion stabilized oxide orhollandiates of manganese, titanium or vanadium.

In this regard, cathodic active materials based on vanadium, such asV₂O₅ are extremely promising candidates for the Mg battery cathode,because vanadium is capable of multiple redox reactions betweenV⁵⁺/V⁴⁺/V³⁺ and V metal. Also, V⁵⁺ as a high valence state is quitestable, which means that it is easy to increase the operating voltage.Various research groups have reported efforts directed to utility ofV₂O₅ as a positive electrode active material.

However, vanadium oxide suffers as an active material for insertion anddeinsertion of magnesium ions because of the strong attraction of theMg²⁺ ion for oxygen of the V₂O₅. This attraction leads to sluggishmagnesium ion diffusion and hinders further magnesiation. Thus, lowcapacity and low rates are obtained with V₂O₅ without furthermodification.

Nanocrystalline V₂O₅ provides improved performance, however, it isconventionally known that nanocrystalline materials have low packingdensity and it is difficult to prepare a cathode having a desired highcapacity and yet have sufficiently small dimensions to be useful insmall scale appliance utility. Thus, the volumetric energy density of acell employing nanocrystalline V₂O₅ would not be acceptable forcommercial applications. Moreover, nanocrystalline materials promoteelectrolyte decomposition due to an extremely high surface area of thenanoparticles.

In an ongoing study of cathode active materials of high energy densityfor utility in a magnesium battery the present inventors have studiedmethods to mitigate the strong force of attraction of the magnesium ionfor V₂O₅. Substitution of sulfur for oxygen in the active material inthe form of an oxysulfide compound has been investigated.

Chen et al (U.S. 2013/0171502) describe a hybrid electrode assemblyhaving a central current collector and on one side of the collector, alayer of a lithium ion intercalation material and on the other side alayer of an intercalation-free material such as a graphene. ConventionalLi intercalation materials are listed in paragraphs [0072], [0104] andin Claim 12. Included in the list are V₂O₅, V₃O₈, sulfur compounds andany mixture thereof.

Bedjaoui et al. (U.S. 2012/0070588) describe a method to package alithium microbattery. In general description of a microbattery, titaniumoxysulfide is described as a cathode active material.

Zhamu et al. (U.S. 2012/0064409) describes a lithium ion battery havingelectrodes containing nano-graphene-enhanced particulate materials.Conventional cathode active materials described include lithium vanadiumoxide, doped lithium vanadium oxide, metal sulfides and combinationsthereof. Explicit disclosure of a mixture of vanadium pentoxide and asulfide glass forming agent is not made, nor is such a materialsuggested.

Gaillard et al. (U.S. Pat. No. 7,695,531) describe a photolithographicmethod to produce an electrolyte thin film for a lithium microbattery.In general description of a lithium microbattery components, titaniumdisulfide, titanium oxysulfide and vanadium oxide are listed as suitablecathode materials.

Gorchkov et al. (U.S. Pat. No. 6,916,579) describe cathode materials fora lithium ion or lithium metal battery which contains a crystallinevanadium oxide and a chalcogenide of sulfur, selenium or telurium. Amixture of vanadium pentoxide and a sulfide glass forming agent is notsuggested.

Mukherjee et al. (U.S. Pat. No. 5,919,587) describe a composite cathodefor an electrochemical cell which is constructed of an electroactivesulfur polymeric material and a transition metal chalcogenide. Othercomponents such as silica, alumina and silicate may be present.Although, cells based on Group I and Group II metals are describedgenerically, explicit disclosure of a magnesium electrochemical cell isnot made. Vanadium pentoxide is disclosed as an example of thetransition metal chalcogenide, however, a mixture of vanadium pentoxideand a sulfide glass forming agent is not made nor suggested. Phosphorouspentasulfide is not disclosed as a component of the cathode activematerial.

Abraham et al. (U.S. Pat. No. 4,934,922) describe a cathode activematerial being a transition metal oxysulfide, preferably molybdenumoxysulfide. Cells based on Group I and Group II metals are describedgenerically, however, the focus is on lithium cells and explicitdisclosure of a magnesium electrochemical cell is not made.

Ouvrard et al. (Journal of Power Sources, 54 (1995) 246-249) describes avanadium oxysulfide compound of formula V₂O₃S.3H₂O as an intercalationmaterial for lithium ions. A lithium electrochemical cell having apositive electrode containing the vanadium oxysulfide is also described.

Aoyagi et al. (U.S. 2012/0164537) describes a positive electrodematerial for a magnesium battery. The cathodic material is a compositeof vanadium oxide, phosphorous oxide, transition metal oxide and otherelements such as alkali metals, sulfur and halogen. The composite isfused at specific temperatures and times to grow a mixed phase systemcontaining vanadium oxide crystallites in an amorphous phosphorous oxidephase. In example I-28 a composition based on V₂O₅, P₂O₅, Fe₂O₃ and LiSis described. A mixture of vanadium pentoxide and phosphorouspentasulfide or any sulfide glass forming agent is not disclosed as acomposition of a cathode active material.

Levi et al. (Chem. Mater. 2010, 22, 860-868) reviews the materialsemployed to date as positive electrode compositions and the problemsassociated with each. V₂O₅ aerogels are discussed; however, nowhere is amixture of vanadium pentoxide and a sulfide glass forming agentdisclosed or suggested.

Doe et al. (U.S. 2012/0219856) describe a series of spinel structurecomposites which serve as chalcogenides for a positive electrode forinsertion and deinsertion of magnesium ion. Vanadyl phosphates aredescribed among many others. However, this reference does not discloseor suggest a mixture of vanadium pentoxide and a sulfide glass formingagent.

Amatucci et al. (Journal of the Electrochemical Society, 148(8)A940-A950 (2001)) (listed in Invention Disclosure) describe a study ofnanocrystalline V₂O₅ as an intercalation material for various cations.Although this reference indicates improvement in performance isnecessary, disclosure or suggestion of mixture of vanadium pentoxide anda sulfide glass forming agent is not made.

Imamura et al. (Journal of the Electrochemical Society, 150(6) A753-A758(2003)) report the synthesis and characterization of a V₂O₅ carboncomposite as a positive electrode material for a magnesium battery, butdoes not disclose or suggest a mixture of vanadium pentoxide and asulfide glass forming agent as a cathode active material.

Imamura et al. (Solid State Ionics, 161 (2003) 173-180) describes theperformance of a V₂O₅ carbon composite as a positive electrode materialfor insertion and desertion of magnesium ion. Nowhere is a mixture ofvanadium pentoxide and a sulfide glass forming agent disclosed orsuggested.

None of the references cited disclose or suggest a mixture of vanadiumpentoxide and a sulfide glass forming agent as a cathode active materialfor a magnesium battery.

Therefore, an object of the present invention is to provide a V₂O₅ basedcathode active material which meets the requirements of a high energymagnesium battery and overcomes the deficiencies of the V₂O₅ formsconventionally known.

Another object of the present invention is to provide a positiveelectrode based on a modified V₂O₅ based material and a magnesiumbattery containing the positive electrode having significantly improvedenergy density and performance in comparison to known magnesiumelectrochemical devices.

SUMMARY OF THE INVENTION

These and other objects are addressed by the present invention, thefirst embodiment of which includes a cathode active material comprisinga composite of vanadium oxide and an inorganic sulfide compound, whereinthe structure of the composite is substantially amorphous and an X-raydiffraction (XRD) analysis does not show a crystalline peak.

In a further embodiment, the present invention includes a cathode activematerial, comprising: nanoparticles of a mechanically milled compositeof a vanadium oxide source and an inorganic sulfide source, wherein thecomposite comprises regions of crystalline morphology embedded in anamorphous matrix.

In embodiments of the invention the vanadium oxide or vanadium oxidesource comprises vanadium pentoxide (V₂O₅) or consists of V₂O₅.

In other embodiments the inorganic sulfide compound or inorganic sulfidesource is at least one selected from the group consisting of P₂S₅, B₂S₃,SiS₂, GeS₂, Al₂S₃ and Ga₂S₃.

Cathodes containing the active materials according to the presentinvention are capable of insertion and release of lithium ions, sodiumions or magnesium ions.

In other embodiments the present invention includes electrochemicalcells containing the cathode active materials according to theembodiments of the present invention.

In another embodiment, the present invention includes a reversiblemagnesium battery comprising: an anode; a cathode; and an electrolyte;wherein the cathode comprises: a current collector; and an activematerial comprising a composite of vanadium oxide and an inorganicsulfide compound, wherein the structure of the composite issubstantially amorphous and an XRD analysis does not show a crystallinepeak.

In another further embodiment, the present invention includes areversible magnesium battery comprising: an anode; a cathode; and anelectrolyte; wherein the cathode comprises: a current collector; and acathode active material, comprising: nanoparticles of a mechanicallymilled composite of a vanadium oxide source and an inorganic sulfidesource, wherein the composite comprises regions of crystallinemorphology embedded in an amorphous matrix.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The presently preferred embodiments, together with furtheradvantages, will be best understood by reference to the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a magnesium battery according to oneembodiment of the present invention.

FIG. 2 shows a schematic electrochemical test cell according to oneembodiment of the present invention.

FIG. 3 shows a XRD pattern of a V₂O₅—P₂S₅ composite according to oneembodiment of the present invention.

FIG. 4 shows Raman spectra of a V₂O₅—P₂S₅ composite according to oneembodiment of the present invention in comparison to V₂O₅ and to P₂S₅.

FIG. 5 shows a transmission electron microscopy (TEM) image of ananoparticle composite according to an embodiment of the invention.

FIG. 6 shows energy dispersive X-ray spectroscopy (EDX) profiles ofslected areas 1, 2 and 3 of the nanoparticle shown in FIG. 5.

FIG. 7 shows an initial linear sweep of voltammogram of a V₂O₅—P₂S₅composite prepared in Example 1 in comparison with the state-of-the-artMo₆S₈.

FIG. 8 shows the first three cycles of a working electrode prepared withan active composite nanoparticle material according to the presentinvention as active material in comparison to an active materialcomposed of ball-milled V₂O₅.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The present inventors are conducting a wide scale study and evaluationof materials which may function as cathode active materials for amagnesium secondary battery. The object of this study is to discovercathode active materials which are readily available, safe andcomparatively easy to handle in a production environment and whichprovide a magnesium battery having high capacity and high workingpotential.

Throughout this description all ranges described include all values andsub-ranges therein, unless otherwise specified. Additionally, theindefinite article “a” or “an” carries the meaning of“one or more”throughout the description, unless otherwise specified.

The inventors have discovered that amorphous compositions of vanadiumoxide and an inorganic sulfide compound are capable of insertion andextraction of metal ions such as lithium, sodium and magnesium,preferably magnesium. When an amorphous composite according to one ofthe embodiments of the present invention is formulated into a cathode, amagnesium battery having high capacity and working potential may beobtained. Initial portions of this work were disclosed in U.S.application Ser. No. 14/260,544, filed Apr. 24, 2014, the disclosure ofwhich is incorporated herein by reference.

Thus, in the first embodiment, the present invention provides a cathodeactive material, comprising: a composite of vanadium oxide and aninorganic sulfide compound, wherein the structure of the composite issubstantially amorphous and an XRD analysis does not show a crystallinepeak.

Upon continued study of the amorphous material obtained via mechanicalmilling methods, the inventors have discovered that under such treatmentconditions a comminuted material containing nanoparticles which are acomposite of regions of amorphous material and regions of crystallinitymay be obtained.

Thus, in a further embodiment, the present invention provides a cathodeactive material, comprising: nanoparticles of a mechanically milledcomposite of a vanadium oxide source and an inorganic sulfide source,wherein the composite comprises regions of crystalline morphologyembedded in an amorphous matrix.

This nanoparticle morphology is shown in FIG. 5 where a compositenanoparticle having a major portion of its volume attributed toamorphous regions as exemplified by circular area 1, while within thenanoparticle, small regions of crystallinity as depicted in circularareas 2 and 3 are also present.

Surprisingly, EDX profiles of the crystalline regions show the presenceof substantially only vanadium and oxygen while the major amorphousregion which appears as a matrix containing the crystal regions containsvanadium, oxygen and the elements of the glass forming inorganic sulfidecompound. Thus, in FIG. 6, the EDX profile of the amorphous area of aV₂O₅/P₂S₅ mechanically milled composite (area 1 of FIG. 5) shows thepresence of phosphorous and sulfur in addition to V and O.

The inventors have surprisingly discovered that the amorphous vanadiumoxide materials according to the various embodiments of the presentinvention as described herein may provide a cathode active materialcapable of a 3V class redox reaction. In preferred embodiments thevanadium oxide or vanadium oxide source is vanadium pentoxide (V₂O₅). Inthe description of the present invention the terms vanadium oxide andvanadium pentoxide may be used interchangeable, unless otherwisespecified. Without being constrained by theory, it is believed that theamorphous structure of the composite matrix contained in each of theembodiments provides many defects and distorted spaces for acceptance ofmetal ions as described above and especially Mg ions. Further, thepresence of the S atoms in the matrix “shields” magnesium ions from thestrong attractive forces of the O atoms and allows for enhanceddiffusion into and out of the matrix. In addition each of the activematerial composites benefits from the capability of the V to function asa redox element by the multiple redox reactions described above.Although not wishing to be limited by theory, the inventors believe theamorphous vanadium oxysulfide region accounts for the electrochemicalactivity of the active material of the present invention.

Amorphorization of the V₂O₅ according to the various embodiments may beconducted employing quenching and mechanical milling methods which areconventionally known. Applicants have discovered that by addinginorganic sulfide compounds as glass forming agents containing at leastone of P₂S₅, B₂S₃, SiS₂, GeS₂, Al₂S₃ and Ga₂S₃ to the V₂O₅ during thepreparation and by careful monitoring of the formation conditions, asubstantially amorphous material or a material containing compositenanoparticles having small regions of crystallinity embedded in anamorphous matrix may be obtained. The degree of crystallinity may bedetermined by control of actual conditions of time, temperature,composition content, quench method and/or mechanical milling mediaemployed in the amorphorization method.

In addition, the relative mol % content of V₂O₅ in the compositematerial affects the performance of a magnesium cell containing thematerial as a cathode active ingredient. With a content of about 35% toabout 95% on a mol % basis, greater reversible redox activity may beobtained. Preferably, the content of V₂O₅ may be from 60 to 90 mol % andmost preferably 67 to 90 mol %. The inventors have determined however,that the most preferred content varies according to the actualcomponents of the composition and the method employed to obtain thecomposite material. Such preferred embodiments may be determined byexperimental methods conventionally known to persons of ordinary skillin the art.

The inorganic sulfide compound or source of inorganic compound maypreferably be selected from the group consisting of P₂S₅, B₂S₃, SiS₂,GeS₂, Al₂S₃ and Ga₂S₃. In one highly preferred embodiment P₂S₅ is usedas the inorganic sulfide compound. Mixtures of these compounds may alsobe employed. In theory any inorganic sulfide compound that forms aglassy composite with V₂O₅ may be employed.

According to the present invention, the description “substantiallyamorphous” means that the material when analyzed by XRD does not showany crystalline peaks.

Thus in one embodiment commercially available V₂O₅ having a minimumpurity of 98%, preferably, a minimum purity of 99% and most preferably,a minimum purity of 99.5% may be physically mixed with an inorganicsulfide compound in a selected mole % ratio. The physical mixture maythen be co-comminuted in any conventional milling apparatus such as aball mill until an XRD spectrum of the milled composite mixture isdevoid of peaks associated with a crystalline material or untilcomposite nanoparticles as shown in FIG. 5 are obtained. The termamorphous vanadium oxide-inorganic sulfide compound composite may beused to represent the various embodiments of the active materialdescribed according to the present invention.

In another embodiment, the physical mixture of the V₂O₅ and inorganicsulfide compound may be heated in an appropriate furnace or oven andquenched by dropping into water or by pressing between two plates orrollers. The amorphous solid solution obtained may then be pulverized.Although the grain size of the pulverlant material is not limited, in apreferred embodiment, the grain size is 10 μm or less, more preferably 5μm or less and most preferably 1 μm or less.

To prepare a cathode the amorphous V₂O₅-inorganic sulfide compoundcomposite according to any of the embodiments of the present inventionmay be mixed with a binder. The binder material is not particularlylimited and any binder recognized by one of skill in the art as suitablemay be employed. Suitable binders may include polyvinylidene fluoride(PVDF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR),and polyimide. Polyvinylidene fluoride may be employed in one preferredembodiment.

In an embodiment of the invention the amorphous V₂O₅-inorganic sulfidecompound composite may be mixed with a carbonaceous material such asgraphite, carbon nanotubes or carbon black.

The amount of binder and carbonaceous material in the cathodecomposition may be no greater than 50% by weight, preferably no greaterthan 30% by weight and more preferably, no greater than 10% by weight.

The cathode according to the present invention may be employed in any ofa lithium battery, sodium battery or magnesium battery. In a preferredembodiment, a reversible magnesium battery having the cathode containingthe amorphous vanadium oxide-inorganic sulfide compound composite isprovided.

The anode of the magnesium battery may be any anode suitable for amagnesium battery, including an anode of magnesium metal or acomposition containing magnesium metal, such as Mg₃Bi₂. The anode activematerial may further include an electrically conductive material and abinder. Examples of electrically conducting materials include carbonparticles, such as carbon black. Example binders include variouspolymers, such as PVDF, PTFE, SBR, and polyimide.

An electrolyte layer may be disposed between the anode and cathode andmay include a separator which helps maintain electrical isolationbetween the positive and negative electrodes. A separator may includefibers, particles, web, porous sheet, or other form of materialconfigured to reduce the risk of physical contact and/or short circuitbetween the electrodes. The separator may be a unitary element, or mayinclude a plurality of discrete spacer elements such as particles orfibers. The electrolyte layer may include a separator infused with anelectrolyte solution. In some examples, for example using a polymerelectrolyte, the separator may be omitted.

The electrolyte layer may include a non-aqueous solvent, such as anorganic solvent, and a salt of the active ion, for example a magnesiumsalt. Magnesium ions provided by the magnesium salt interactelectrolytically with the active material(s). An electrolyte may be anelectrolyte including or otherwise providing magnesium ions, such as anon-aqueous or aprotic electrolyte including a magnesium salt. Theelectrolyte may include an organic solvent. Magnesium ions may bepresent as a salt or complex of magnesium, or as any appropriate form.

An electrolyte may include other compounds, for example additives toenhance ionic conductivity, and may in some examples include acidic orbasic compounds as additives. An electrolyte may be a liquid, gel, orsolid. An electrolyte may be a polymer electrolyte, for exampleincluding a plasticized polymer, and may have a polymer infused with orotherwise including magnesium ions. In some examples, an electrolyte mayinclude a molten salt. In one aspect, the electrolyte may include phenylmagnesium chloride (PhMgCl⁺) with aluminum trichloride (AlCl₃ ⁻) intetrahydrofuran (THF), magnesium monocarbonane [Mg(CB₁₁H₁₂)₂] intetraglyme (TEGDME) or magnesium perchlorate [Mg(ClO₄)₂] in acetonitrile(ACN). In a preferred embodiment, the electrolyte may be Mg(ClO₄)₂ inACN.

The cathode active material may be present as a sheet, ribbon,particles, or other physical form. An electrode containing the cathodeactive material may be supported by a current collector.

A current collector may include a metal or other electrically conductingsheet on which the electrode is supported. The current collector may beformed of carbon, carbon paper, carbon cloth or a metal or noble metalmesh or foil.

FIG. 1 shows an example of one configuration of a rechargeable magnesiumcell 5. The cell 5 includes a positive electrode 10 including theamorphous V₂O₅-inorganic sulfide compound composite according to theinvention as the cathode active material, an electrolyte layer 12, anegative electrode 14, a cathode current collector 16, a negativeelectrode housing 18, a positive electrode housing 20 including an inertlayer 21, and a sealing gasket 22. The electrolyte layer 12 may includea separator soaked in electrolyte solution, and the positive electrode10 may be supported by the cathode current collector 16. In thisexample, the negative electrode 14 includes an active material ofmagnesium metal.

FIG. 2 shows a schematic diagram of a three electrode cell which may beuseful for evaluation and characterization of the cathode activematerials of the present invention. In FIG. 2 the cell is constructedwith a glass vial having a silicone cap. The reference electrode (R.E.)is a Ag/Ag⁺ electrode consisting of a Ag wire in a reference solution of0.01 M AgNO₃ and 0.1 M tetrabutylammonium perchlorate (TBAP) inacetonitrile (ACN). The working electrode (W.E.) is constructed of an 80mesh stainless steel screen upon which a layer of the active material tobe tested is formed. The anode (C.E.) is constructed of a magnesiumfoil. The electrolyte is a 1 M Mg(ClO₄)₂ in ACN. A test cell as shownschematically in FIG. 2 may be useful to conduct cyclic voltammetry,impedance and charge/discharge testing. Such testing may be conducted inan argon atmosphere by placing the test cell in a glove box.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

EXAMPLES

The invented materials were prepared by ball milling of the startingmaterials (V₂O₅ and P₂S₅) under a rotation speed of 370 rpm for 20 h inAr atmosphere. The ZrO₂ balls and pot were used for ball millingsynthesis. After ball milling, the samples were collected under Aratmosphere. The invented sample was mixed with Acetylene black(Denkikagaku Kogyo, HS-100) and PVDF (Kureha, KF9305) inN-methylpyrrolidone (NMP) solution to prepare a homogeneous cathode ink.The obtained ink was cast on an 80 mesh stainless steel and then driedat 120° C. under vacuum.

FIG. 3 shows the XRD pattern of V₂O₅—P₂S₅ material prepared in the ballmilling process. No crystalline peak was observed by XRD, which meansthat the obtained material became amorphous.

FIG. 4 shows the Raman spectra of V₂O₅—P₂S₅ material in comparison withP₂S₅ and V₂O₅. The addition of P₂S₅ provides a different chemicalenvironment from V₂O₅ and P₂S₅. Also, the characteristic peak for Sinclusion was observed at around 400-500 cm⁻¹.

FIG. 5 shows a TEM image of a V₂O₅—P₂S₅ ball-milled sample showing ananoparticle as a composite of an amorphous matrix with embeddedcrystalline regions. Area 1 represents a typical amorphous morphologywhich was observed as the occupying the major portion of thenanoparticle volume in this sample. Areas 2 & 3 show regions of V₂O₅crystals embedded in the amorphous matrix. EDX profiles of the selectedareas of the V₂O₅—P₂S₅ sample in FIG. 5 are shown in FIG. 6. In theamorphous sample, the elements of V, O, P and S were clearly detected,while only the elements of V and O were observed in the crystals.

FIG. 7 shows the initial linear sweep voltammograms of V₂O₅—P₂S₅material in comparison with the state-of-the-art Chevrel Mo₆S₈ materialin the conventional based electrolytes (1M Mg(ClO₄)₂ in ACN). Scan ratewas 0.1 mV/sec and the operation temperature was 25° C. The current wascathodically swept and was normalized by the maximum current observed ineach voltammogram. The potential was normalized by the Ag referenceelectrode. The observed working potential of the V₂O₅-inorganic sulfidecompound composite was higher than that of the Chevrel phase(comparative example 1). Also, according to our experiments, comparabledischarge capacity was obtained in different Mg battery electrolytes. Itis also noted that 0V vs. Ag/Ag⁺ in potential corresponded toapproximately 2.5 V vs. Mg/Mg²⁺.

FIG. 8 shows the first three cycles of nanoparticle composite V₂O₅—P₂S₅sample as a working electrode in the three electrode cell set-up shownin FIG. 2. The initial cycle of the ball milled V₂O₅ sample (comparativeexample 2) is shown for comparison. The current density was 0.1 mA/cm²and the operation temperature was 25° C. The potential was normalized bythe Ag reference electrode. It was obvious that the observed capacity ofthe V₂O₅—P₂S₅ sample was higher than that of the V₂O₅ sample. Althoughcapacity fading was observed mainly due to a side reaction from theanode side, the cell was repeatedly cycled.

In conclusion, the synthesized amorphous vanadium oxide-inorganicsulfide compound according to the embodiments of the present inventionshowed significantly improved electrochemical properties than thebenchmarked Chevrel Mo₆S₈ material and referenced V₂O₅ material,suggesting higher energy density may be obtained according to theembodiments of the invention in comparison to known state-of-the-artcathode materials.

Numerous modifications and variations on the present invention arepossible in light of the above description and examples. It is thereforeto be understood that within the scope of the following Claims, theinvention may be practiced otherwise than as specifically describedherein. Any such embodiments are intended to be within the scope of thepresent invention.

The invention claimed is:
 1. A cathode active material, comprising:nanoparticles of a mechanically milled composite of a vanadium oxidesource and an inorganic sulfide source, wherein the mechanically milledcomposite comprises regions of crystalline morphology embedded in anamorphous matrix, and the inorganic sulfide source is at least oneselected from the group consisting of P₂S₅, B₂S₃, SiS₂, GeS₂, Al₂S₃ andGa₂S₃.
 2. The cathode active material of claim 1, wherein the amorphousmatrix comprises at least vanadium, oxygen and sulfur.
 3. The cathodeactive material of claim 1, wherein the region of crystalline morphologycomprises substantially only vanadium and oxygen as determined by EDXanalysis.
 4. The cathode active material of claim 1, wherein thevanadium oxide source consists of vanadium pentoxide.
 5. The cathodeactive material of claim 1 wherein the mechanically milled composite isa ball-milled composite.
 6. The cathode active material of claim 1,wherein the inorganic sulfide source is B₂S₃.
 7. The cathode activematerial of claim 6, wherein the inorganic sulfide source is P₂S₅. 8.The cathode active material of claim 7, wherein the amorphous matrixconsists of vanadium, oxygen, phosphorous and sulfur.
 9. A cathode,comprising: a current collector: and the cathode active material ofclaim
 1. 10. The cathode of claim 9, wherein the amorphous matrix of themechanically milled composite nanoparticle comprises at least vanadium,oxygen and sulfur.
 11. The cathode of claim 9, wherein the vanadiumoxide source of the mechanically milled composite nanoparticle consistsof vanadium pentoxide.
 12. The cathode of claim 9, wherein the inorganicsulfide source of the mechanically milled composite nanoparticle isB₂S₃.
 13. The cathode of claim 9, wherein the vanadium oxide source ofthe mechanically milled composite nanoparticle comprises vanadiumpentoxide.
 14. The cathode of claim 13, wherein the vanadium pentoxideis V₂O₅, and a content of the V₂O₅ is from 67 to 90 mol % of the totalmol % of the V₂O₅ and the inorganic sulfide.
 15. The cathode of claim 9,wherein the inorganic sulfide source is P₂S₅.
 16. The cathode of claim15, wherein the amorphous matrix consists of vanadium, oxygen,phosphorous and sulfur.
 17. The cathode of claim 9, wherein the vanadiumoxide source is vanadium pentoxide and the inorganic sulfide source isP₂S₅.
 18. The cathode of claim 17, wherein the vanadium pentoxide isV₂O₅, and a content of the V₂O₅ is from 67 to 90 mol % of the total mol% of the V₂O₅ and the P₂S₅.
 19. An electrochemical cell comprising: ananode; the cathode of claim 9; and an electrolyte; wherein the anode andcathode are capable of absorbing and releasing a metal ion selected fromthe group consisting of a lithium ion, a sodium ion and a magnesium ion.20. A magnesium battery comprising the electrochemical cell of claim 19wherein the anode and cathode are capable of absorbing and releasing themagnesium ion.
 21. The magnesium battery of claim 20, wherein thevanadium oxide source of the mechanically milled composite nanoparticleconsists of vanadium pentoxide.
 22. The magnesium battery of claim 20,wherein the vanadium oxide source of the mechanically milled compositenanoparticle comprises vanadium pentoxide.
 23. The magnesium battery ofclaim 22, wherein the vanadium pentoxide is V₂O₅, and a content of theV₂O₅ is from 67 to 90 mol % of the total mol % of the V₂O₅ and theinorganic sulfide.
 24. The magnesium battery of claim 20, wherein theinorganic sulfide source of the mechanically milled compositenanoparticle is.
 25. The magnesium battery of claim 24, wherein theinorganic sulfide source is P₂S₅.
 26. The magnesium battery of claim 20,wherein the vanadium oxide source is vanadium pentoxide and theinorganic sulfide source is P₂S₅.
 27. The magnesium battery of claim 26,wherein the vanadium pentoxide is V₂O₅, and a content of the V₂O₅ isfrom 67 to 90 mol % of the total mol % of the V₂O₅ and the P₂S₅.